bus-proxy: augment credentials from /proc for cmdline update

dbus1 does not provide cmdline, so we have to augment our credentials from
/proc to beautify the bus-proxyd cmdline. We dont use this for anything
but beautification, so there shouldn't be any problems due to /proc
pid-recycling races.

This fixes bus-proxyd to no longer display 'xxxxxxxxxxxxxxxxxxxxxxxxxxx'
in its cmdline.